1. Field of the Invention
This invention relates to flow field analysis. More specifically, the invention is a schlieren system and method for recording density changes in a flow field surrounding a moving object.
2. Description of the Related Art
Since the early days of flight, scientists have worked to examine the aerodynamic flow field surrounding an aircraft. Typically, scientists are interested in the effects of turbulence and shockwaves on aircraft. However, the invisible nature of air has turned such studies into a branch of research that is carried out with models of the aircraft in wind tunnel environments.
Initially, wind tunnel testing involved injecting streams of smoke into the wind tunnel in order to study the flow around models of aircraft. However, smoke visualization techniques proved inadequate for airplane speeds reaching into the region of compressible flow where shockwaves begin to form. In order to examine compressible flow (as well as any changes in the optical density of any gas or clean liquid), schlieren photographic systems were developed in which shockwaves (or any density changes) appear as dark bands extending from the object or vehicle under examination within a wind tunnel.
A basic schlieren system is shown in the diagrammatic view of FIG. 1 where it is referenced generally by numeral 100. In brief, schlieren system 100 consists of point light source 102 emitting light represented by rays 103, two lenses 104 and 106 on either side of test section 108 of a wind tunnel (not shown) housing model 109, image recording film 110, and mask edge 112 located between lens 106 and film 110 for preventing part of the light passing through lens 106 from impinging on film 110. Specifically, if the air flow in test section 108 is left undisturbed, lens 106 simply reconstructs an image of point source 102 and an image of model 109 for reproduction on film 110. However, if the density of the air moving about model 109 is disturbed, then some of light rays 103 passing around model 109 are bent or refracted. In the example shown, mask edge 112 blocks the light that is bent downward as indicated by dashed line 103a while light that is bent upward continues on. Thus, the ultimate image appearing on film 110 includes an image of model 109 and light and dark bands extending from the image of model 109 to indicate density changes in the air flow about model 109. Such schlieren systems are well known in the art and are disclosed in greater detail by W. Merzkirch, "Flow Visualization", Academic Press, New York, 1974, pages 86-102.
Although producing useful information, wind tunnels are limited since not all flow features of a full scale aircraft in flight can be simulated. On the other hand, in-flight measurements have been limited to using clouds of smoke or tracer particles to either selectively show the outline of an aircraft's features or as a detection medium for velocimeters. Unfortunately, these in-flight techniques require complex, on-board instrumentation and are limited to aircraft flying at moderate speeds, e.g., subsonic speeds of less than 743 miles per hour.